(1) Field of the Invention
The present invention relates to carbon, based electrodes and, in particular, to carbon based electrodes for the electrochemical reduction of sulfur or oxygen.
(2) Description of Related Art
It is well known in the electrochemical field that carbon is not an electrocatalyst for the reduction of sulfur (Allen, P. L. and Hickling, A., Trans. Faraday Soc., 53 (1957) 1626). Various workers have endeavoured to find good elecrocatalysts for the reduction of sulfur which enable the electrode to operate at a current density of greater than 20 mAcmxe2x88x922 at an over potential of less than 50 mV. To date the only materials which have been found to be sufficiently active are metal sulfides. However, the materials with the highest activity do not show good long term stability and are expensive to produce.
A separate problem occurs when carrying out the sulfide/polysulfide redox reduction which is described, for example, in U.S. Pat. No. 4,485,154. The current density at an electrode carrying out the sulfide/polysulfide redox reaction is limited by the combined effects of the reactions in solution and slow electrochemical reaction kinetics. Many authors (Lessner, P. M., McLaron, F. R., Winnick, J. and Cairns, E.J., J. Appl. Electrochem., 22 (1996) 927-934, Idem., ibid. 133 (1986) 2517) have proposed a metal or metal sulfide deposited on a high surface area electrode (e.g. an expanded metal mesh) to overcome these effects by providing a high interfacial area per unit volume and an electrocatalyst surface. The catalytic electrode surface layers of Ni, Co or Mo metals, or the sulfides of these metals, achieve only a modest current density of 10 to 20 mAcmxe2x88x922 at an over voltage of 50 mV.
Electrodes which are surfaced with carbon are recognized to be two to three orders of magnitude worse than the catalytic electrode surface layers of Ni, Co or Mo metals, or the sulfides of these metals. Accordingly, a 50 mV overvoltage would be achieved only at current densities of 0.1 to 1 mAcmxe2x88x922. For example, overvoltagaes of 300 to 500 mV are encountered at 40 mAcmxe2x88x922 even when the electrode surface is coated with a high surface area carbon (see U.S. Pat. No. 4,069,371 and U.S. Pat. No. 4,177,204).
It would be advantageous to be able to use carbon based electrodes for the electrochemical reduction of sulfur in the sulfide/polysulfide redox energy storage system because a carbon based electrode would not suffer from degradation due to inter-conversion between various sulfide phases. We have now developed a technique for making carbon based electrodes which are electrocatalytically active for the sulfur reduction process. Such electrodes are also suitable for the electrochemical reduction of oxygen.
Accordingly, the present invention provides a carbon based electrode for the electrochemical reduction of sulfur or oxygen, which comprise an electrode core and, in electrical contact therewith, a structure comprising a porous particulate activated carbon bonded with a polymeric binder material, characterised in that the structure is at least 1 mm thick, in that the particulate activated carbon is prepared from a lignocellulosic material and has the following properties:
i) a particle size in the range of from 200 to 850 xcexcm;
ii) a pore volume of from 0.45 to 1.0 cm3 per gram;
iii) a surface area in the range of from 800 to 1500 m2/g;
and in that the binder is used in an amount not exceeding 25% by weight based upon the mixture of activated carbon and binder material.